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The use of venom across a wide variety of taxa is an example of convergent evolution. It is difficult to conclude exactly how this trait came to be so intensely widespread and diversified. The multigene families that encode the toxins of venomous animals are actively selected, creating more diverse toxins with specific functions. Venoms adapt to their environment and victims and accordingly evolve to become maximally efficient on a predator's particular prey (particularly the precise ion channels within the prey). Consequently, venoms become specialized to an animal's standard diet.[2]

Venom is widely distributed taxonomically, being found in both invertebrates and vertebrates; in aquatic and terrestrial animals; and among both predators and prey. The major groups of venomous animals are described below.

In insects such as bees and wasps, the stinger is a modified egg-laying device — the ovipositor. In Polistes fuscatus, the female continuously releases a venom that contains a sex pheromone that induces copulatory behavior in males.[12] In Polistes exclamans, venom is used as an alarm pheromone, coordinating a response with from the nest and attracting nearby wasps to attack the predator.[13] In Dolichovespula arenaria, the observed spraying of venom out of their sting has been seen from workers in large colonies.[14] In other cases like Parischnogaster striatula, the venom is applied all over their body as an antimicrobial protection.[15] The venom from Agelaia pallipes has inhibitory effects on processes like chemotaxis and hemolysis which can lead to organ failure.[16]

Many caterpillars have defensive venom glands associated with specialized bristles on the body, known as urticating hairs, which can be lethal to humans (e.g., that of the Lonomia moth), although the venom's strength varies depending on the species.[17]

Bees synthesize and employ an acidic venom (apitoxin) to cause pain in those that they sting to defend their hives and food stores, whereas wasps use a chemically different alkaline venom designed to paralyze prey, so it can be stored alive in the food chambers of their young. The use of venom is much more widespread than just these examples. Other insects, such as true bugs and many ants, also produce venom. At least one ant species (Polyrhachis dives) has been shown to use venom topically for the sterilisation of pathogens.[18]

Some 450 species of snake are venomous.[24]Snake venom is produced by glands below the eye (the mandibular gland) and delivered to the victim through tubular or channeled fangs. Snake venoms contain a variety of peptide toxins, including proteases, which hydrolyze protein peptide bonds, nucleases, which hydrolyze the phosphodiester bonds of DNA, and neurotoxins, which disable signalling in the nervous system.[27] Snake venom causes symptoms including pain, swelling, tissue necrosis, low blood pressure, convulsions, hemorrhage (varying by species of snake), respiratory paralysis, kidney failure, coma and death.[28] Snake venom may have originated with duplication of genes that had been expressed in the salivary glands of ancestors.[29][30]

Extensive research on platypuses shows that their toxin was initially formed from gene duplication, but data provides evidence that the further evolution of platypus venom does not rely as much on gene duplication as once was thought.[40] Modified sweat glands are what evolved into platypus venom glands. Although it is proven that reptile and platypus venom have independently evolved, it is thought that there are certain protein structures that are favored to evolve into toxic molecules. This provides more evidence as to why venom has become a homoplastic trait and why very different animals have convergently evolved.[41]